Binary Acid Strength Within a Group

A binary acid is an acid that consists of hydrogen bonded to another nonmetal. The strength of a binary acid depends on the strength of the bond between the hydrogen atom(s) and the other atom. As the strength of the bond decreases, the compound can release $\text{H}^+$ ions more easily in solution, so the acid is stronger.

Within a group, the strength of the binary acids tends to increase as one goes down the periodic table. This is because the atomic radius of the other atom increases, causing the distance between it and the hydrogen atoms to be larger, which results in weaker bonds.

The best example of this is group $17.$ Hydrofluoric acid $\text{HF}$ ($K_a = 8\times 10^{-4}$) is weaker than hydrochloric acid $\text{HCl}$ ($K_a = 1\times 10^6$), which is weaker than hydrobromic acid $\text{HBr}$ ($K_a = 1\times 10^9$), which is weaker than hydroidoic acid $\text{HI}$ ($K_a = 3\times 10^9$). (Note that $\text{HF}$ is a weak acid, while the other three are strong acids.)